The present invention relates to a display-integrated type tablet device for use in a personal computer, a word processor, or the like.
As a means for inputting a handwritten letter or a figure into a computer or a word processor, there has been put into practical use a display-integrated type tablet device which is formed by laminating an electrostatic induction type tablet on a liquid crystal display and is capable of receiving an input of a letter or a figure into its electrostatic induction type tablet as if the letter or figure were written on a paper by writing implements.
However, in such a display-integrated type tablet device, electrodes are viewed as a grating on the display screen due to a difference in reflectance between a portion having an electrode and a portion having no electrode, which has been a cause of degrading the quality of an image displayed on the liquid crystal display screen.
FIGS. 1 and 2 show display-integrated type tablet devices proposed lately (Japanese Patent Laid-Open Publication No. 5-53726 and Japanese Patent Application No. 4-320545) each as a tablet free of the above-mentioned drawback, while FIG. 3 shows an explanatory view of an image display period and a coordinate detection period in each of those display-integrated type tablet devices. It should be noted that Japanese Patent Application No. 4-320545 shown in FIG. 2 is not laid-opened yet and thus is not a prior art.
In each of those display-integrated type tablet devices, electrodes concurrently serve as image display electrodes of a liquid crystal display (LCD) and as coordinate detection electrodes of an electrostatic type tablet device. There are provided in one frame period a coordinate detection period when designated coordinates on the tablet are detected and an image display period when an image is displayed as shown in FIG. 3, where the coordinate detection and image display are time-sharingly effected.
FIG. 1 shows a display-integrated type tablet device having a duty type LCD panel 1. Referring to FIG. 1, the LCD panel 1 is constructed by interposing liquid crystals between n number of common electrodes Y.sub.1 through Y.sub.n (an arbitrary common electrode represented by Y hereinafter) and m number of segment electrodes X.sub.1 through X.sub.m (an arbitrary segment electrode represented by X hereinafter) which are arranged at right angles to each other, in which each portion where a common electrode Y and a segment electrode X cross each other constitutes each pixel. In other words, n.times.m dot pixels are arranged in matrix in the LCD panel 1.
In the display-integrated type tablet device, various display control signals from a display control circuit 5 are selected by a switching circuit 4 to be supplied to a common electrode drive circuit 2 and a segment electrode drive circuit 3 in the image display period. Then drive pulses of a common electrode drive signal are applied to the common electrode Y by the common electrode drive circuit 2. Meanwhile, drive pulses of a segment electrode drive signal are applied to the segment electrode X by the segment electrode drive circuit 3.
In the coordinate detection period, various detection control signals from a detection control circuit 6 are selected by the switching circuit 4 to be supplied to the common electrode drive circuit 2 and the segment electrode drive circuit 3. By the common electrode drive circuit 2, scanning pulses of common electrode scanning signals y.sub.1 through y.sub.n (an arbitrary common electrode scanning signal represented by y hereinafter) are successively applied to the common electrodes Y.sub.1 through Y.sub.n. Meanwhile, by the segment electrode drive circuit 3, scanning pulses of segment electrode scanning signals x.sub.1 through x.sub.m (an arbitrary segment electrode scanning signal represented by x hereinafter) are successively applied to segment electrodes X.sub.1 through X.sub.m.
Referring to FIG. 3, the coordinate detection period 301 is separated into an x-coordinate detection period 301a and a subsequent y-coordinate detection period 301b. In the x-coordinate detection period, scanning pulses of the segment electrode scanning signal x are successively applied to the segment electrode X. In the y-coordinate detection period, scanning pulses of the common electrode scanning signal y are successively applied to the common electrode Y. The image display period 302 occurs before the coordinate detection period 301.
In the course of the above-mentioned operations, a voltage is induced at a designation coordinate detection pen (referred to merely as the "detection pen" hereinafter) 8 due to a stray capacitance between the segment electrode X or the common electrode Y and a detection electrode of the detection pen 8 attributed to the application of the scanning pulses. Then an x-coordinate detection circuit 10 and a y-coordinate detection circuit 11 detect the x-coordinate or the y-coordinate at the point pointed by the detection pen 8 based on an induction voltage signal obtained by amplifying in an amplifier 9 an induction voltage induced at the detection pen 8 and a timing signal from a control circuit 7.
FIG. 2 shows a display-integrated type tablet device having an active matrix type LCD panel 21. In the LCD panel 21, n number of gate bus line electrodes G.sub.1 through G.sub.n (an arbitrary gate bus line electrode represented by G hereinafter) and m number of source bus line electrodes S.sub.1 through S.sub.m (an arbitrary source bus line electrode represented by S hereinafter) are insulated and arranged at right angles to each other. In a position where the gate bus line electrode G and the source bus line electrode S cross each other is provided a TFT (Thin Film Transistor) 25. A pixel is composed of a pixel electrode 24 connected to the TFT 25, an opposite electrode (not shown) opposite to the pixel electrode 24, and liquid crystals filled in between both the electrodes. In other words, n.times.m dot display pixels are arranged in matrix in the LCD panel 21.
In the coordinate detection period, the detection pen 8 is placed on an input surface of the LCD panel 21 and a voltage induced at the detection electrode of the detection pen 8 due to an electrostatic coupling between the detection electrode of the detection pen 8 and the gate bus line electrode G or the source bus line electrode S is detected to obtain the coordinate at the tip end of the detection pen 8 by means of the x-coordinate detection circuit 10 or the y-coordinate detection circuit 11 in the same manner as in the display-integrated type tablet device having the duty type LCD panel 1.
Therefore, when a letter or a figure is input to the LCD panel 1 or the LCD panel 21 in the display-integrated type tablet device having the duty type LCD panel 1 or the display-integrated type tablet device having the active matrix type LCD panel 21, the trace of the tip end of the detection pen 8 can be displayed on the LCD panel 1 or the LCD panel 21 by sliding the detection pen 8 on the LCD panel 1 or the LCD panel 21 with the detection pen 8 put in contact with the input surface to obtain the coordinates at the tip end of the detection pen 8 based on an induction voltage signal detected by the detection pen 8 and outputting display data for displaying a dot in the position of the coordinates at the tip end from the display control circuit 5.
In the above case, the tip end portion of the detection pen 8 is covered with a tip end resin coat 33 having an insulation capability so that the input surface of a protection panel 31 for protecting the LCD panel 1 or the LCD panel 21 is not scratched by the detection electrode 32 as shown in FIG. 4 which illustrates in detail the tip end portion of the detection pen 8.
However, the above-mentioned display-integrated type tablet device having the duty type LCD panel 1 or the active matrix type LCD panel 21 has the following problems.
In the coordinate detection period, a scanning pulse of an electrode scanning signal is applied alternately to the segment electrode X or the source bus line electrode S and the common electrode Y or the gate bus line electrode G to induce a voltage at the detection electrode 32 having a high input impedance provided at the tip end of the detection pen 8 based on the scanning pulse to thereby detect the coordinates at the tip end of the detection pen 8 according to the timing of generation of the induction voltage.
However, the voltage induced at the detection electrode 32 of the detection pen 8 is very weak. Particularly in the display-integrated type tablet device having the duty type LCD panel 1 as shown in FIG. 1, an electric field exerted from the electrode placed in a lower position (segment electrode X in FIG. 1) among the segment electrode X and the common electrode Y is shielded by the electrode placed in the upper position (common electrode Y in FIG. 1). Therefore, a leakage electric field leaking through narrow gaps between the electrodes in the upper position must be detected in scanning the electrodes in the lower position. The induction voltage obtained in scanning the electrodes in the lower position is a very weak voltage of several millivolts.
In the display-integrated type tablet device having the active matrix type LCD panel 21 as shown in FIG. 2, the source bus line electrode S and the gate bus line electrode G are used as scanning electrodes for driving the TFT 25. However, since each of the source bus line electrode S and the gate bus line electrode G has an electrode width of several tens microns, the induction voltage detected by the detection pen 8 is very weak.
As described above, the voltage induced at the detection electrode 32 of the detection pen 8 is very weak in the display-integrated type tablet device having the duty type LCD panel 1 and the display-integrated type tablet device having the active matrix type LCD panel 21. Therefore, the induction voltage is very susceptible to external noise.
Among various sources of noise, there is noise caused by electric charges due to friction between the tip end resin coat 33 of the detection pen 8 and the protection panel 31 in the time of writing a letter or drawing a figure on the LCD panel 1 or the LCD panel 21 by sliding the detection pen 8 on the protection panel 31.
Since the frictional electric charges gradually reduces by a leak current through the tip end resin coat (dielectric material) 33 of the detection pen 8 and the protection panel 31, the noise caused by the frictional electric charges generated between the tip end resin coat 33 and the protection panel 31 is not problematic on normal conditions.
However, when the display-integrated type tablet device is placed in condition where the environmental humidity is abnormally low, the dielectric material of the tip end resin coat 33 of the detection pen 8 has a great resistance. Therefore, in the case where the voltage generated by the frictional electric charges surpasses the reduction, the voltage of the frictional electric charges increases, and when exceeding the critical point, it causes an instantaneous electric discharge toward a portion having less electric charges. The above-mentioned phenomenon also occurs in continuing handwriting with the detection pen 8. In such a case, since the voltage at the portion having electric charges greatly changes in a moment due to the electric discharge, a spike-shaped noise voltage is induced at the detection electrode 32 of the detection pen 8 and input as an error detection signal to the coordinate detection circuits 10 and 11.
In a very rare case, when an electric appliance operating at a high frequency and a high voltage is put close to the input surface of the LCD panel 1 or the LCD panel 21, the voltage from the electric appliance is detected as noise by the detection electrode 32 of the detection pen 8 to exert bad influence on the detection of coordinates at the tip end of the detection pen 8.
FIG. 5A shows the waveform of an induction voltage induced at the detection electrode 32 of the detection pen 8 in the time of scanning the segment electrode X placed in the lower position in the coordinate detection period in the duty type LCD panel 1 shown in Fig. 1, where no external noise exists. FIG. 5B shows a binary signal obtained by binarizing the induction voltage having a waveform as shown in FIG. 5A by a slice voltage "e.sub.s " in a comparator (not shown).
In the above case, when the scanning is executed only once in the x-coordinate detection period, one pulse is generated in the resulting binary signal based on the induction voltage induced at the detection electrode 32 of the detection pen 8 as shown in FIG. 5B.
However, in the case where the detection electrode 32 of the detection pen 8 detects external noise in the x-coordinate detection period, a plurality of pulses are generated in the binary signal even when the segment electrode X is scanned only once. Consequently, an x-coordinate signal representing a coordinate that is different from the true coordinate at the tip end of the detection pen 8 and detected by the x-coordinate detection circuit 10 based on the pulse attributed to the external noise is also output.
Therefore, when a letter or a figure is input to the LCD panel 1 or the LCD panel 21, an image attributed to the external noise is displayed erroneously in a position that is not traced by the tip end of the detection pen 8 other than the true trace of the tip end of the detection pen 8 on the LCD panel 1 or the LCD panel 21.